Acid base or generator

ABSTRACT

A method for generating high purity acid or base in an aqueous stream for use as an eluent for chromatography and, particularly, ion chromatography. For generating a base for anion chromatography, the aqueous stream is directed through a cation exchange bed having a first strongly acidic and a second weakly acidic portions. An electrical potential is applied to the bed. Cations on the bed electromigrate into the aqueous stream while hydroxide ions are electrolytically generated to form a base-containing eluent. Anions to be detected and the generated eluent flow through a chromatographic separator. The chromatographic effluent flows past a detector.

CROSS-REFERENCE TO RELATED APPLICATION

This is a division of application Ser. No. 09/006,096 filed Jan. 13,1998 now U.S. Pat. No. 6,036,921. Application Ser. No. 09/006,096 is acontinuation-in-part of H. Small U.S. patent application Ser. No.08/783,317, filed Jan. 15, 1997 now abandoned.

BACKGROUND OF THE INVENTION

The present invention relates to a method and apparatus for generationof a high purity acid or base particularly for use as a chromatographyeluent.

In liquid chromatography, a sample containing a number of components tobe separated is directed through a chromatography separator, typicallyan ion exchange resin bed. The components are separated on elution fromthe bed in a solution of eluent. One effective form of liquidchromatography is referred to as ion chromatography. In this knowntechnique, ions to be detected in a sample solution are directed throughthe separator using an eluent containing an acid or base and thereafterto a suppressor, followed by detection, typically by an electricalconductivity detector. In the suppressor, the electrical conductivity ofthe electrolyte is suppressed but not that of the separated ions so thelatter may be detected by the conductivity detector. This technique isdescribed in detail in U.S. Pat. Nos. 3,897,213, 3,920,397, 3,925,019and 3,956,559.

There is a general need for a convenient source of high purity acid orbase for use, such as an eluent for liquid chromatography and,particularly, for ion chromatography. In one technique, described inU.S. Pat. No. 5,045,204, an impure acid or base is purified in an eluentgenerator while flowing through a source channel along a permselectiveion exchange membrane which separates the source channel from a productchannel. The membrane allows selective passage of cations or anions. Anelectrical potential is applied between the source channel and theproduct channel so that the anions or cations of the acid or base passfrom the former to the latter to generate therein a base or acid withelectrolytically generated hydroxide ions or hydronium ions,respectively. This system requires an aqueous stream of acid or base asa starting source or reservoir.

There is a particular need for a pure source of acid or base which canbe generated at selected concentrations solely from an ion exchange bedwithout the necessity of an independent reservoir of an acid or basestarting aqueous stream. There is a further need for such a system whichcan be continuously regenerated. Such need exists in chromatography, andspecifically ion chromatography, as well as other analyticalapplications using acid or base such as in titration, flow injectionanalysis and the like.

SUMMARY OF THE INVENTION

In accordance with the present invention, a method and apparatus hasbeen provided for generating acid or base in an aqueous stream, such aswater alone or in combination with additives (e.g. ones which react withthe acid or base or with the sample). The system provides an excellentsource of high purity acid or base for use as an eluent forchromatography and, particularly, ion chromatography.

Referring first to a system in which a base is generated for anionchromatographic analysis, an aqueous stream is directed through aflowthrough cation exchange bed including exchangeable cations. Anelectrical potential is applied between an anode and a cathode inelectrical communication with an inlet and an outlet portion of the bed,preferably near the inlet and outlet, respectively, of the bed. Cationson the bed electromigrate into the aqueous stream while hydroxide ionsare electrolytically generated to form a base-containing eluent. Aliquid sample stream containing anions to be detected together with thegenerated eluent flow through a chromatographic separator in which theanions are separated. The chromatographic effluent flows past a detectorin which separated anions are detected. In one embodiment, betweenseparation and detection, the separated anion stream flows through aflowthrough second cation exchange bed including exchangeable cationsand hydronium ions with no electrical potential being applied. Thesecond bed serves as a suppressor with hydronium ions in the bed beingdisplaced by ion exchange with the cations of the base to convert thebase to weakly ionized form for conductivity detection.

In a preferred embodiment, after completion of the desired number ofcycles, flow through the first and second cation exchange beds isreversed by appropriate valving for the next sample stream. Anelectrical potential of the type described with respect to the first bedis applied to the second bed while the potential is discontinued in thefirst bed. Thus, the second bed generates eluent and the first bedsuppresses conductivity of the base.

In another embodiment, the second cation exchange bed is disposed afterthe detector. In this instance, the second bed serves no suppressionfunction. Instead, it serves as a sink or trap to retain the cations ofthe base and the eluent. By appropriate valving, flow may be reversedthrough the first and second beds with potential applied in the secondbed but not the first bed. During the reversal setting of the valving,the second bed generates the base.

Since hydrogen and oxygen gases are generated in the ion exchange bedswhich could interfere with detection, it is preferable to pressurize thechromatographic effluent prior to detection, such as by use of a flowrestrictor.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic flow diagram of an ion chromatography system usingan eluent generator according to the invention, without recycle.

FIG. 2 is an expanded view of an eluent generator according to theinvention.

FIGS. 3 and 4 are schematic flow diagrams of an ion chromatographysystem using eluent generation according to the invention, with recycle,illustrating flow using two different valve settings.

FIGS. 5 and 6 are schematic flow diagrams of a liquid chromatographysystem using eluent generation according to the invention, with recycle,illustrating flow using two different valve settings.

FIG. 7 is a chart of conductance of KOH vs. current.

FIGS. 8-17 are chromatograms of various experiments performed inaccordance with the present invention.

FIGS. 18-21 are comparative exhaustion profiles and operating voltagefor single-bed and dual-bed KOH generators.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The method and apparatus for generation of acid or base according to thepresent invention will first be described to supply eluent for ionchromatography. For simplicity, although applicable to anion or cationanalysis, the system will be described for the analysis of anions on anion exchange resin packed bed form of an exchange bed. In this instance,the first cation exchange bed generates a base such as an alkali metalhydroxide, typically sodium or potassium. Conversely for analysis ofcations, the eluent generated is an acid such as hydrochloric acid.

The system is applicable to the generation of eluent for liquidchromatography forms other than ion chromatography. For example, it isapplicable to liquid chromatography using an ultraviolet (UV) detector.The eluent may be in a form (e.g. salt) other than a pure acid or base.Thus, the term “aqueous stream” includes pure water or water with suchadditives. Also, the terms “eluent comprising a base”, “eluentcomprising an acid”, an “acid” or a “base” mean an aqueous streamincluding acid or base generated according to the invention regardlessof the form it takes on mixing with other reagents present in theaqueous stream.

Referring specifically to FIG. 1, a simplified ion chromatographyapparatus is illustrated. The system includes a source of an aqueousstream such as deionized water reservoir 10 which is pumped bychromatography pump 12 to column 14, serving to generate acid or base,through sample injection valve 16 into chromatographic separator 18packed with a chromatographic separation medium, typically an ionexchange resin packed bed. Alternative other forms of separation mediummay be used such as a porous hydrophobic chromatographic resin withessentially no permanently attached ion exchange sites such as describedin EPA publication 180,321, incorporated herein by reference.

A column 20 is in fluid communication with separator 18, serving tosuppress the conductivity of the base in the effluent from separator 18,but not the conductivity of the ions injected through sample injectionport 16.

The effluent from suppressor 20 is directed through a flow-throughconductivity cell 22 for detecting the resolved ions in the effluentfrom suppressor 20. A suitable data system, not shown, is provided inthe form of a conventional conductivity detector for measuring thesuppressor effluent in the conductivity cell 22. In conductivity cell22, the presence of an ionic species produces an electrical signalproportional to its concentration. Such signal is typically directedfrom cell 22 to a conductivity meter (not shown) forming part of a datasystem permitting direct detection of the concentration of the separatedionic species. With the exception of column 14, such ion chromatographysystems are well known, e.g. as illustrated in U.S. Pat. Nos. 3,897,231; 3,920,397; 3,925,019 and 3,956,559 incorporated herein by reference.

The system also includes means for pressurizing the effluent prior todetection to minimize adverse effects of gases (hydrogen and oxygen)generated in the eluent generator 14 as will be described hereinafter.As illustrated in FIG. 1, such pressurizing means comprise a flowrestrictor 24 downstream of conductivity cell 22 to maintain the ionchromatography system under pressure.

FIG. 2 illustrates in more detail column 14 of the present invention asa base or acid generator. Column 14, typically in the form of a hollowcylinder 26, contains ion exchange bed 28. Column 14 is shownschematically, and so cylinder 26 is not illustrated with its top andbottom walls or with the inlet and outlet liquid couplings which can beof a conventional type used for packed bed suppressor columns orchromatography separator columns sold by Dionex Corporation. For thegeneration of base, the bed is of a strongly acidic cation exchangetype, preferably a packed ion exchange resin bed. Column 14 alsoincludes electrodes 30 and 32 in the upstream and downstream portions ofthe cation exchange bed. “Inlet portion” means the inlet side half ofthe bed and “outlet portion” means the outlet side half of the bed.Alternatively, one or both electrodes may be physically separated fromthe cation exchange bed so long as the electrodes are in electricalcommunication with the adjacent bed. For example, an electricallyconductive screen may separate one or both of the electrode from thebed.

As illustrated, power supply 33 is connected so that electrode 30 hasanode polarity and electrode 32 a cathode polarity. Preferably, thepower supply 33 includes a variable outlet potential for reasons whichwill be described.

The illustrated electrodes 30 and 32 are suitably in the form of porousinert metal disks or frits (e.g. formed of platinum), preferablydisposed near the inlet and outlet respectively of the column, servingalso as supports holding the resin particles of the packed bed in closecontact. By “near the inlet and outlet” is meant placing the electrodeswithin about 10 to 20% of the inlet and outlet of the bed, respectively,and preferably at the inlet and outlet. The electrodes may be in directcontact with such portions of the bed or may be adjacent to suchportions of the bed in electrical contact therewith. Also, theelectrodes may take forms other than porous disks (e.g. rings, screensor probes) so long as they provide good contact with the ion exchangebed, preferably by direct contact.

As used herein, the terms “anion exchange bed, cation exchange bed orion exchange bed” refer to a flow-through bed of ion exchange materialthrough which the aqueous stream flows. The term “cation” excludeshydronium ion and the term “anion” excludes hydroxide ion. Because ofits ready availability and known characteristics, a preferred form ofion exchange bed is a packed ion exchange resin bed. It is importantthat the resin particles be packed in the bed tightly enough to form acontinuous ion bridge or pathway for the flow of anions or cationsbetween the electrodes. Also, there must be sufficient spacing for theaqueous stream to flow through the bed without undue pressure drops. Forthis purpose, the ion exchange resin packing of Dionex Corporationpacked bed suppressor columns (Dionex ASC cation exchange resin andDionex CSC anion exchange resin) may be used.

In the present system, the source or reservoir of cations for generatingthe base is a cation exchange bed alone or combined with a second cationexchange bed as described below. This is to be contrasted with theeluent generator of U.S. Pat. 5,045,204 in which the reservoir of theacid or base is a preexisting aqueous stream of acid or base whichprovides the cations or anions which pass transversely through themembrane into the eluent product flow stream. In the present invention,the source of cations or anions is the ion exchange bed and the acid orbase is generated during the flow of the aqueous stream through the bed.The present cation exchange bed extends in a network transverse to theflowing aqueous stream and provides an electrical path for cations incontact with both the anode and the cathode. Thus, the definition of ionexchange bed excludes a membrane suppressor structure.

In one form of the invention the entire bed comprises ion exchangematerial commonly classified as strongly acidic cation type or stronglybasic anion type. This embodiment will be referred to as the “single-bedgenerator”. The desired capacity for the single-bed ion exchangematerial is that which provides a sufficient capacity of generating acidor base for the desired analysis system. It is preferred to use ionexchange materials that have ion exchange capacity ranging from about0.4 to about 2.4 meq/mL. However, ion exchange materials with eitherhigher or lower capacity may also be used. Examples of suitable cationexchange resins include Dowex 50W, 200-400 mesh resins and Dionex ASCcation exchange resin. The examples of suitable anion exchange resinsinclude Dowex 1, 200-400 mesh resin and Dionex CSC anion exchangeresins.

In another form of the invention, the ion exchange bed includes anupstream bed portion and an adjacent downstream bed portion. The ionexchange capacity of the upstream portion is substantially higher thanthe ion exchange capacity of the downstream bed portion. The upstreambed portion normally contains substantially more (e.g. 10 times or more)ion exchange material than the downstream portion. Usually the upstreamportion is of the same cross-sectional area but substantially longerthan the downstream bed portion, depending on the system. Thus, theupstream bed portion may be about 10 to 20 longer than the downstreambed portion in a typical system.

The above system will be termed the “dual-bed generator”. In thedual-bed base generator, the upstream bed portion preferably comprises astrongly acidic cation exchange resin (e.g., a sulfonated resin in K⁺form) and the downstream bed portion preferably comprises a weaklyacidic cation exchange form (e.g., a carboxylated resin also in K⁺form). Similarly, for the generation of acid, the upstream bed portionis of the strongly basic anion exchange type (e.g., a resin withquaternary amine functional groups) and the downstream bed portion is ofthe weakly basic anion exchange type (e.g. a resin with tertiary orsecondary amine functional groups). For either type of downstream bedportion the capacity preferably is in the range of from about 0.4 toabout 2.4 meq/mL. However, ion exchange materials with either higher orlower capacity may also be used. For the downstream bed portion,suitable cation exchange resins include Dionex CS12A resin and Bio-Rex70 resin, and examples of anion exchange resins include AG 3-X4.

The advantage of using the dual-bed approach can be seen by viewing whatmay happen in a single-bed base generator, all of the strongly acidiccation exchange resin types. For example, in a KOH generator column ofthis type, H⁺ ions generated at the anode displace K⁺ ions in the bed,and the displaced K⁺ ions combine with OH⁻ ions generated at the cathodeto produce KOH in the carrier solution (i.e. deionized water) beforeleaving the generator column. H⁺ ions migrate faster through the resinbed than K⁺ ions under the applied field. Once the zone of H⁺ ionsreaches the cathode, some of the applied current is not utilized forgeneration of KOH, and the concentration of KOH generated becomes lowerthan what is prescribed by the applied current and carrier flow rate(i.e., the quantitative linear relationship between the KOHconcentration and the applied current becomes invalid). Therefore, theuseful capacity of a single-bed KOH generator column (defined as theamount of K⁺ ions producing a constant concentration of KOH when theapplied current and the carrier flow rate are constant) is about 40percent of the total K⁺ capacity of the generator column.

The dual-bed KOH generator column is an approach to increase the usefulcapacity of a KOH generator column. In the dual-bed KOH generatorcolumn, once H⁺ ions reach the bed of the weakly acidic resin, theirmigration through the resin bed is significantly slowed down because oftheir higher affinity to the weakly acidic functional groups. On theother hand, the migration of K⁺ ions through the resin bed is littleaffected. Therefore, more K⁺ ions are able to reach the cathode to formKOH before the arrival of H⁺ ions at the cathode, and thus the usefulcapacity of the KOLI generator column is increased. In the dual-bed KOHgenerator column, once H⁺ ions reach the weakly acidic resin bed, theapplied voltage needed to maintain the constant current will increasedue to the development of the less conductive protonated zone in theweakly acidic resin bed.

The exchangeable cations or anions must also be sufficiently watersoluble in base or acid form for use at the desired concentrations.Suitable exchangeable cations are metals, preferably alkali metals suchas sodium, potassium, lithium and cesium. Potassium is particularlyeffective because it is a common, relatively inexpensive species and, ofthe two common cations, sodium and potassium, a cation exchange resin inpotassium form has the lower electrical resistance. Other suitablecations are quaternary ammonium cations such as tetramethyl ammonium,tetraethyl ammonium and tetrabutyl ammonium. For cation analysis,suitable exchangeable anions include chloride, sulfate and methanesulfonate.

For a packed bed, the higher the cross-linking of a resin the higher itscapacity (expressed as milliequivalents per ml. of column); therefore,higher cross- linked resins give more compact generators andsuppressors. This is desirable. However, the higher the cross-linking ofa resin, the less it deforms when packed in a column. Some deformationis desirable in that it improves the area of contact between resin beadsthus lowering the electrical resistance of the packed bed. Lowerresistance means that a particular level of current may be attained at alower applied voltage; this, in turn, leads to less heating of the bedwhile carrying current, a desirable feature.

Bead deformation is favored by lowering the degree of cross linking.But, resin of very low cross-linking (say 1 to 2%) is so deformable thatat certain flow rates the deformation can lead to undesirably highpressure across the bed. In summary, a wide range of cross-linking canbe used. Resins of moderate cross-linkage are to be preferred, typicallyin the range of 4 to 16% divinyl benzene for styrene divinyl benzenepolymer beads.

Other forms of ion exchange beds can be used such as a porous continuousstructure with sufficient porosity to permit flow of an aqueous streamat a sufficient rate for use as an eluent for chromatography withoutundue pressure drop and with sufficient ion exchange capacity to form aconductive bridge of cations or anions between the electrodes. One formof structure is a porous matrix or a sponge-like material with aporosity of about 10 to 50% permitting a flow rate of about 0.1 to 3ml/min without excessive pressure drop. Another suitable form is a rollof ion exchange film (e.g. in a configuration of such a roll on aspindle disposed parallel to liquid flow). Electrodes would be placed ateach end of the roll which could be textured to provide an adequate voidchannel.

For the production of pure base (e.g. potassium hydroxide), high purity(deionized) water is pumped to the bed in an accurate and controllableflow rate by a typical high pressure chromatography pump. To generatebase while water is flowing through the bed, a D.C. potential is appliedbetween the inlet and outlet of the bed using the inlet porous inertmetal disk as the anode and the outlet porous inert metal disk as thecathode. Water splitting takes place at both electrodes. The anodereaction in the upstream portion of the bed is

H₂O−2e ⁻→2H⁺+1/2 O₂  (1)

During this reaction, hydronium ions are produced. The hydronium ionspass into the cation exchange resin by electromigration displacing theexchangeable cations (e.g., K⁺ ions) ahead of them. This displacementtakes place along the length of the bed eventually leading to theproduction of base (e.g., KOH) in the flowing aqueous stream in thevicinity of the cathode. The arriving cations on the column couple withthe hydroxide ions produced in the aqueous stream to produce a base. Thehydroxide ions are produced in the following cathodic reaction.

2 H₂O+2e ⁻→2OH⁻+H₂  (2)

According to the invention, the concentration of the generated base isreadily controlled by correspondingly controlling (1) the flow rate ofthe aqueous stream through the bed, (2) the electrical current throughthe bed, and (3) both current and flow simultaneously.

The electrode reactions produce electrolysis gases, hydrogen and oxygen,which are carried forward into the chromatography system. If these gasesare produced in significant volume relative to the liquid flow, theirpresence can be detrimental to chromatographic operation. This potentialproblem can be eliminated by application of Boyle's law. The system canbe operated over a wide range of pressures, e.g., ambient to about 1500psi above ambient. An elevated pressure (e.g. 250 to 500 psi ) ispreferred so that the gases are compressed to a volume that isinsignificant compared to the flow of the aqueous liquid stream. Thepressure necessary to accomplish this depends on the volume of gasesproduced. One mode of elevating the pressure is to connect a flowrestrictor 24 such as a fine bore coiled tubing downstream of thedetector (e.g. three meters of 0.005 in. ID). This elevates the pressurethroughout the chromatography system upstream of the detector. Thispressure is higher than that used for conventional ion chromatographysystems. In the present system, it is preferable to construct theconductivity cell to be capable of withstanding a pressure of 1500 psior more above ambient pressure. A lower pressure of 250 to 500 psi couldbe used in most conditions. Such system pressure may be high enough tointerfere with effective use of membrane suppressors. Accordingly, forthis reason and others to be described hereinafter, a packed bedsuppressor is preferable.

By using a constant flow of water while with constant current, thedevice of FIG. 2 generates a constant concentration of base. Therelationship of current, water flow rate, and base concentration may beillustrated by the following discussion.

Assume that the current through the bed is a constant 10 milliamps. Therate of charge (ion) transfer to the cathode is therefore 10millicoulombs per second or 600 millicoulombs per minute. From FaradaysLaw, it is known that 96,500 coulombs of charge produce one equivalentof ions. Therefore, rounding 96,500 to 100,000, it is calculated that600 millicoulombs per minute produce 6 microequivalents per minute ofbase (potassium hydroxide) at the cathode. Consequently, if the flowrate of water is a constant 1 ml/min, the concentration of potassiumhydroxide leaving the cathode will be 0.006 M. Higher flow rates ofwater will yield proportionately lower concentrations of potassiumhydroxide while higher currents will yield proportionately higherconcentrations.

One of the advantages of the present system is the ability tocontrollably produce very low concentrations of base required for aparticular application. Control may be accomplished solely by adjustingthe current.

The aqueous stream in source 10 may be high purity deionized water.Ilowever, for some forms of chromatography, and particularly ionchromatography, it may be desirable to modify the source with anadditive which reacts with the base generated at the cathode to produceeluents of varying potency. For the production of a base, such wellknown additives include a source of carbonic or boric acid, phenol,cyanophenol, and the like. For the production of acid, such additivesinclude m-phenylene diamine, pyridine and lysine. The aqueous stream ispumped at rates determined by the analytical process to be used. For ionchromatography, typical flow rates of 0.1 to 3 mL/min are employed.

The current (voltage) requirements of a generator depend on (a) theeluent strength required; (b) the diameter of the column; (c) the lengthof the column; (d) the electrical resistance of the resin; and (e) theflow rate of the aqueous phase. For example, a fresh 4×150 mm column ofpotassium form of Dowex 50W×8 run at 8 milliamps and a flow rate of 1mL/min generates approximately 0.005M potassium hydroxide. The voltagerequired initially when the bed is fresh is about 160 volts.

As the generator exhausts, that is, as it converts to the hydroniumform, its resistance drops and a progressively lower voltage will berequired to maintain the current at 8 milliamps.

A sodium form resin which has a higher resistance than the potassiumform will require a somewhat higher voltage to produce a certain currentall other factors being the same.

To produce a certain current, the voltage applied to a generator will beproportional to its length and inversely proportional to its crosssectional area, all other factors being the same.

It is normally important to maintain a constant current it is directlyrelated to the concentration of acid or base. A feedback loop may beprovided to assure sufficient voltage to deliver the predetermined,constant current (e.g. 10 milliamps). Thus, the current is monitored andwhen the resistance changes, the potential is correspondingly changed bythe feedback loop. Therefore, the voltage is a slave to the reading ofthe current. Thus, it is preferable to supply a variable outputpotential system of this type (e.g. one sold under the designationElectrophoresis Power Supply EPS 600 by Pharmacia Biotech.

In the simplified system illustrated in FIG. 1, no form of recycle orother regeneration is illustrated. The recycle system described belowwill serve to recycle the cations on column 14 for continuous use. Inone form of application, if column 14 has sufficient capacity, it couldbe discarded after long term use and replaced when so many exchangeablecations are used that a constant concentration of base is no longergenerated under a constant current and a carrier flow rate.Alternatively, the generator could be regenerated by passage of acid orbase respectively through column 14 which is off line. However, apreferred form regeneration is by recycle as described below.

The above flow system of FIG. 1 has been described with respect to anion chromatography system. However, generation by column 14 is broadlyapplicable to a variety of other analytical systems. For example, itcould be used for a liquid chromatography system using a detector otherthan an ion conductivity detector in which suppressor 20 is not used. Anexample of detector suitable for such a liquid chromatography system isa UV detector.

The ion chromatography system of the present invention has beendescribed with respect to the generation of base. It is also applicableto the generation of an acid with the appropriate modification of theion exchange beds and polarity of the electrodes. In this instance, ananion exchange bed is used in column 14 to generate acid. Analogous tothe generation of base, the anode is disposed in the downstream upstreamportion of the bed, preferably at the outlet, while the cathode isdisposed in the upstream portion, preferably at the inlet. The reactionsat the anode and cathode are as illustrated in equations (1) and (2)above, respectively. Here, the exchangeable anion (e.g. chloride) isdisplaced at the entrance of the bed by the hydroxide ions generated atthe.cathode. Such anions move along the bed toward the anode wherehydronium ions are generated. The anions electromigrate from the bedinto the aqueous stream near the anode to form an acid with thehydronium ions.

The foregoing parameters regarding the type of ion exchange bed,potential applied, and the like apply in analogous fashion to thegeneration of acid as to the generation of base. Suitable anion exchangeresins include Dowex 1×8 200-400 mesh and Dionex CSC anion exchangeresin. Suitable exchangeable anions used for liquid chromatography maybe employed here including chloride, sulfate, methane, sulfonate,phosphate, acetate, phthalate, and benzoate to produce the correspondingacids.

Ion Chromatography Recycle Embodiment

Referring to FIGS. 3 and 4, a preferred embodiment of the system of thepresent invention for generating base is illustrated in which eluent iscontinuously regenerated. A system with the valve settings illustratedin FIG. 3 functions the same way as the schematic system of FIG. 1. Tosimplify the figures, the power supply is only illustrated in the figurein which the potential is applied. Like parts in FIG. 1 will bedesignated with like numbers in FIGS. 3 and 4.

Referring to FIG. 3, the aqueous stream from source 10 is pumped by pump12 through valves 40 and 42 into column 14 which generates base asdescribed above. The power supply is connected to apply an electricalpotential to anode 30 and cathode 32 at the inlet and outlet,respectively, thereof. A cation exchange bed in column 14 generates basewhich passes as an eluent through valves 44 and 52, into separatorcolumn 18. A sample stream containing anions to be detected is injectedthrough port 16 into separator column 18 in the eluent where the anionsare separated. The effluent from column 18 flows through valves 40 and48, through column 20, through valves 50 and 52 and then throughconductivity cell 22 in which the separated anions are detected, throughflow restrictor 23, and to waste. In this instance, column 14 generatesbase while column 20 serves as a conventional suppressor to convert thebase to weakly ionized form. During suppression, column 20 which isoriginally in the hydronium form, is being converted to the cation form.The hydronium form of the column is converted to cation as in aconventional suppressor from the inlet side first. Since no electricalpotential is applied to column 20, the power supply is not shown in FIG.3.

Referring to FIG. 4, the valving is reversed so that the flow of FIG. 3is discontinued and the flow direction of FIG. 4 commences. As anoverview, flow through the injection port, separator column,conductivity cell and restrictor is the same direction. However, flow isreversed through columns 14 and 20. In this instance, column 14 servesas a conventional suppressor with no electrical potential being appliedand so the power supply to column 14 is not shown. In FIG. 4, anelectrical potential is applied by power supply 60 connected to anode 62at the inlet portion of column 20 and cathode 64 at the outlet end ofcolumn 20.

Flow in the valve setting of FIG. 4 is from aqueous stream source 10through pump 12, valves 40, 46 and 50, and then through column 20 in anopposite direction to flow through that column in FIG. 3. Column 20which had been partially converted from the hydronium form to the cationform is used a source of cations to generate base in the same manner ascolumn 14 when it was totally in the cation form at the very beginningof operation illustrated in FIG. 3. Flow from the outlet of column 20flows through valves 48, 50, and 52, through sample injector port 16 inwhich the sample is injected, separator column 18 in which the anionsfrom the next sample are separated, valves 40, 42 and 44, and thenthrough column 14 functioning now as a suppressor. From there, theeffluent flows through valves 42, 44 and 52 through conductivity cell22, restrictor 23 and to waste.

While specific valving is illustrated for accomplishing recycle, it isapparent to those of skill in the art that other valving may be employedas long as flow can be reversed between columns 14 and 20.

The parameters and conditions for operating columns 14 and 20 aredirectly analogous to the operation of columns 20 and 14, respectively,in the valve setting of FIG. 3. Column 20 serves as the eluentgenerator, while column 14 serves as the suppressor. Thus, it ispreferable that the ion exchange beds in columns 20 and 14 have similarcharacteristics, such as column dimensions and ion exchange capacities,because their functions are reversed in FIG. 4 compared to FIG. 3.Specifically, column 20 becomes a base generator while column 14 becomesa suppressor.

In this system with the valve setting of FIG. 4, cations trapped in thesuppressor are used as the source of cations for generating the base.The system provides a source of base for the eluent within the ionexchange bed itself, by reversing the flow and operation of columns 20and 14 at appropriate intervals. Suitably, this point of reversal isaccomplished when approximately 20 to 80% of the eluent generatorexchange bed has converted from the cation to the hydronium form or whenapproximately 20 to 80% of the suppressor has converted from thehydronium form to the cation form. The system is preferably adjusted sothat these conversions proceed at substantially the same rates. Thesystem is designed to run a predetermined number of samples (e.g. one)to as many as ten or more, with the valve setting of FIG. 3 beforeconverting to the valve setting of FIG. 4.

On startup, each bed, generator and suppressor can be converted to thehalf exhausted condition, and thereafter can operate with each bedoscillating about 10% or less on either side of this condition. Theseconditions permit the analysis of at least one sample between theswitching of modes. Preferably, they permit the analysis of a largenumber of samples between mode switching: for example, samplescontaining low affinity analytes which would typically require verydilute eluents and place a concomitantly lower demand on the generatorand suppressor.

Over a long time, cations from the sample could build up on columns 14and 20 which may eventually interfere with perpetual regeneration of theeluent generator. This could be addressed by periodically flushing thesystem with an aqueous acid or base regeneration solution.

Recycle for Liquid Chromatography Systems Other than Ion ChromatographySystem

The above system of FIGS. 3 and 4 is illustrated with columns 14 and 20serving dual functions, generating eluent and suppressing theconductivity of the eluent prior to conductivity detection. The systemof continuous regeneration also applies to other liquid chromatographysystems in which there is no need for, and where it may be undesirableto use a suppressor between the separator column and the detector. Inthis instance, a column of the same type as columns 14 and 20 could beplaced downstream of the detector serving as a trap for the cations oranions which could be recycled using appropriate valving for continuousgeneration of base or acid. Here, in a second setting of the valving,the flow proceeds in the opposite direction between these two columnswith appropriate modifications of the application of the electricalpotential. A system of this type is illustrated in FIGS. 5 and 6.

Referring specifically to FIGS. 5 and 6, another preferred embodiment ofthe system of the present invention for generating base is illustratedin which eluent is continuously regenerated. Here, the system is liquidchromatography using a detector such as a UV-Vis detector. The secondcolumn is downstream of the detector because no suppression prior todetection is performed. To simplify the figures, the power supply isonly illustrated in the figure in which the potential is applied.

Referring to FIG. 5, the aqueous stream (deionized water) is pumped fromsource 70, pump 72, valves 74 and 76 into column 78 which generates baseas described above. The power supply is connected to apply an electricalpotential to anode 80 and cathode 82 at the inlet and outlet,respectively, thereof. A cation exchange bed in column 78 generates basewhich passes as an eluent through valves 84 and 86, into separatorcolumn 88. The sample stream containing a target analyte to be detectedis injected through sample injection port 90 and is carried by theeluent into separator column 88 where the target analytes are separated.The effluent from column 88 flows past a suitable detector 92 such as aUV-Vis detector or other non-destructive detector in which the separatedanalytes are detected. After the detector, the effluent flows throughvalves 74 and 94 into column 96 which serves as a sink or trap forcations in the base which had been generated in column 78. Column 96 isoriginally in the hydronium ion form and is converted to the cation formin the same manner as in suppressor 20 described with respect to FIG. 3.However, in this instance, no suppression function is performed becausecolumn 96 is downstream from detector 92. Since no electrical potentialis applied to column 96 in this valve setting, the power supply is notshown in FIG. 5. The effluent from column 96 flows through valves 98 and86 through flow restrictor 100 and to waste. Restrictor 100 serves thesame function as restrictor 22 in FIGS. 3 and 4.

Referring to FIG. 6, the valving is modified so that the flow of FIG. 5is discontinued and flow in the direction of FIG. 6 commences. As anoverview, flow through the injection port, separator column, detectorand restrictor is in the same direction. Ilowever, flow is reversedthrough column 78 and 96. In this valve position, no potential is beingapplied to column 78 which serves to trap cations in the same manner ascolumn 96 in FIG. 5. Thus, no power supply for column 78 is illustrated.In FIG. 6, an electrical potential is applied by power supply 102connected to anode 104 and cathode 106 at the inlet and outlet ends,respectively of column 96.

Flow in the valve setting of FIG. 6 is from source 70 through pump 72,valves 74, 94 and 98, and then through column 96 in opposite directionto flow in that column in FIG. 5. Column 96 which had been partiallyconverted from the hydronium ion form to the cation form is used as asource of cations to generate base in the same manner as in column 78when it was in cation form at the beginning of operation illustrated inFIG. 5. The aqueous stream flows from the outlet of column 96 throughvalves 94, 98 and 86 through sample injector port 90 in which the sampleis injected, and into separator column 88 in which the target analytesare separated. From there, the aqueous stream flows past detector 92 inwhich the target analytes are detected and then through valves 76 and 84to column 78 in opposite direction to flow through that column in FIG.5. Column 78 now serves as a trap or sink for cations in the same manneras column 96 had served as described in FIG. 5. The effluent from column78 flows through valves 76, 84 and 86 through restrictor 100 and towaste.

While specific valving is illustrated for accomplishing recycle for thisliquid chromatography application, it is apparent to those of skill inthe art that other valving may be employed so long as flow can bereversed between columns 78 and 96.

The same principles of substantially matching the parameters andconditions for operating columns 78 and 96 apply here as described withrespect to the analogous column 20 and 30 with respect to FIGS. 3 and 4.

In order to further illustrate the present invention, the followingexamples are provided.

EXAMPLE 1

Operation of Single-Bed NaOH Generator

This example illustrates a base (metal hydroxide) generator of the typeillustrated in FIG. 2. A column (3-mm ID×175-mm length) was filled withDowex 50W 200-400 mesh Na⁺ form and each end fitted with a porous Ptdisk. In one experiment, while pumping water through the column at 2mL/min, the voltage was changed in steps with the current passingthrough the column. Conductance of the effluent was recorded in Table 1.FIG. 7 shows the excellent linear dependence of conductance on current.

From the conductance one can calculate the concentration of sodiumhydroxide produced. One can also calculate the concentration of sodiumhydroxide to be expected from the current and water flow rate. In oneexperiment the current was 1.9 mA and the specific conductance measuredfor the NagH solution generated was 859 μs.cm. The conductancecorresponded to a concentration of 0.00345 N while one would expect aconcentration of 0.0037 N from the current and flow rate. The agreementbetween these two values is very good.

EXAMPLE 2

Operation of a Single-Bed KOH Generator

In this experiment, water was pumped at 1 ml/min through a bed ofpotassium form resin carrying a current of 10 mA. The effluent wascollected, titrated and found to be 0.0051 N in base. The currentthrough the bed was then reduced to 5 mA and the effluent was determinedto be 0.0025 N. This experiment confirmed the linear dependence ofeffluent concentration on current and illustrates the ability toprecisely control the concentration of eluent produced by controllingthe current through the column.

EXAMPLE 3

Use of a Single-Bed KOH Generator in Ion Chromatography

The ion chromatographic system, illustrated schematically in FIG. 1, wasassembled to test a KOH generator in an IC application. FIG. 8 is thechromatogram obtained for a mixture of fluoride, chloride and nitrate(0.0001 M in each) when the generator was run at 5 mA. FIG. 9 is thechromatogram of the mixture when the generator was run at 10 mA. Theelution data are summarized in Table 2.

Ions of lower affinity (fluoride, acetate and formate) were alsoseparated on the same system. The separation of FIG. 10 is remarkable ina number of ways:

1. that the voltage applied to the generator was only 6 V;

2. that the current was only 0.2 mA; and

3. that the concentration of KOH being produced by the generator wasonly 0.0001 N.

The last feature is noteworthy in that it illustrates how easy it is toprepare potassium hydroxide of such low concentration and preserve itfrom contamination with carbonate. The challenge of accomplishing thisby conventional means, that is by diluting a concentrated solution ofKOH while preventing contamination by CO₂, underlies the efficacy andconvenience of the electrochemical eluent generator.

TABLE 1 Voltage (volts) Current (mA) sp. conductance μs · cm 0 0 <5 501.27 87 100 2.7 189 150 4.32 306 200 6.4 444 300 11.9 859

TABLE 2 Ion Current (mA) t_(E) (min.) t_(E) − t_(void) Void — 0.90 0fluoride 10 0.95 0.05 chloride 10 1.27 0.37 nitrate 10 2.22 1.32 sulfate10 4.70 3.80 fluoride 5 1.00 0.10 chloride 5 1.67 0.77 nitrate 5 3.522.62 sulfate 5 15.7 14.8

EXAMPLE 4

Eluent Recycle

An experimental system assembled to implement the eluent recycle conceptis illustrated in FIGS. 3 and 4. The system consisted of a reservoir ofdeionized water, pump, KOH generator column, separation column,suppressor column, conductivity cell, and flow restrictor that wereinterconnected through three Dionex BF-2 double-stack, 4-way, 2-positionvalves. In a working embodiment, a single-bed KOH generator column (4-mmID×50-mm length) was packed with a Dionex 18 μcm cation exchange resinin the K⁺ form, and a Dionex AS-11 column (4-mm ID×250-mm length) wasused as the separation column. The three 4-way valves were automaticallyswitched between their two positions using a computerized controller. Apower supply was connected to the KOH generator column usingcomputer-controlled relays.

In the valve position of FIG. 3, column 20 (the KOH generator column)was supplied with current to produce KOH, and Column 30 was used as thesuppressor column. The system was operated in this valve position untilColumn 20 was converted about 50 percent to the H⁺ form by H⁺ ionsgenerated at the anode, and Column 30 was converted about 50 percent tothe K⁺ form by K⁺ ions released from Column 20. At this point, bothColumns 20 and 30 had acquired the ability to generate and suppresshydroxide, and the roles of Columns 20 and 30 were reversed by switchingthe position of the three 4-way, 2-position valves to alter thedirection of the eluent flow. The system was then switched between thevalve position of FIGS. 3 and 4 for every sample run. In this manner,the KOH eluent was recycled.

In one experiment, the KOH generator column was supplied with a constantcurrent of 18 mA (the applied voltage was 30V) and the eluent flow ratewas maintained at 0.5 mL/min (yielding 22 mM KOH). FIG. 11 shows anexample of the separation of fluoride, chloride, sulfate, nitrate andphosphate obtained after the system was switched automatically betweenthe positions of FIGS. 3 and 4 for more than 60 sample injections. Theseresults successfully demonstrated that the isocratic ion chromatographicseparations of anions can be achieved by implementing the eluent recycleconcept.

In another experiment, the current supplied to the KOH generator columnwas changed from 0.5 mA to 34 mA at a rate of 1.0 mA/min to generate agradient of KOH from 0.62 mM to 42 mM at a flow rate of 0.5 mL/min. FIG.12 shows an example of the separation of fluoride, chloride, nitrate,sulfate and phosphate obtained with the system operated in the valvepositions of FIGS. 3 and 4.

FIG. 13 shows an example of the separation of a mixture of 27 anionswith the system operated in the valve positions of FIGS. 3 and 4. Theseresults successfully demonstrated that the eluent recycle concept can beutilized to achieve gradient ion chromatographic separations of anions.

EXAMPLE 5

Single-Bed Hydrochloric Acid Generator

A 4-mm ID×150-mm long column of the type illustrated in FIG. 2 wasfilled with Dowex 1×8, 200-400 mesh, in the chloride form. The columnwas equipped with porous platinum bed supports (electrodes). Thechromatography system illustrated schematically in FIG. 1 was assembledto test the HCl generator.

In one test, water was pumped through the system at 1 ml/min, while avoltage (251 V) was applied to the generator giving a current of 2.1 mA.Ten microliters of sample was injected. The sample contained Li⁺ at 5mg/mL; Na+, K⁺, and Mg²⁺ at 20 mg/mL; NH⁺ at 40 mg/mL; and Ca²⁺ at 100mg/mL. The run was terminated when the alkali metals and ammonium hadeluted and the chromatogram is shown in FIG. 14.

In another test, the generator was polarized at 311 V which generated acurrent of 8.1 mA and a similar sample injected. In this instance therun was continued until magnesium and calcium had eluted. Thatchromatogram is illustrated in FIG. 15.

EXAMPLE 6

Single-Bed Methane Sulfonic Acid Generator

A quantity of Dowex 1×8, 200-400 mesh, chloride form was washed with acopious amount of 1.0 N NaOH in order to convert the resin tosubstantially the hydroxide form. The resin was then neutralized withmethane sulfonic acid (MSA) to convert it to the methane sulfonate form.

Using the same procedure as the HCl generator, a 4-mm ID×150-mm lengthcolumn was filled with the Dowex 1×8 methane sulfonate resin, equippedwith porous Pt electrodes and tested.

In one test, the MSA generator was pumped with water a 1 ml/min,polarized (275 V) to give a current of 8.1 mA and the six cationstandard sample (see Example 5) injected. The chromatogram is shown inFIG. 16.

In another test, the generator was polarized at 88V giving a current of1.9 mA and the six cation standard injected. This run was terminatedwhen ammonium and the alkali metals had eluted. The chromatogram isshown in FIG. 17.

EXAMPLE 7

Sponge-like Bed Generator

In this example, a flow-through, sponge-like ion exchange bed is formed.Styrene and divinyl benzene are copolymerized in the presence of anappropriate catalyst and a porogen. A porogen is an added materialwhich, when removed after the polymerization is complete, creates amacroporosity in the polymerized structure. This porosity should be suchthat it provides for ready flow of liquids through the polymer phasewhile at the same time providing adequate area of contact between thepolymer and the liquid phase. The porogen can be a finely divided solidwhich can be easily removed by dissolution in acid or base, e.g.,calcium carbonate or silica, or it can be a solvent which is rejected bythe polymer as it forms and is subsequently displaced by another solventor water. Examples of suitable liquid porogens include an alcohol suchas dodecyl alcohol, e.g. used in the manner described in AnalyticalChemistry, Vol. 68, No. 2, pp. 315-321, Jan. 15, 1996.

After the porogen is removed, the polymer is sulfonated by commonlyknown sulfonating agents such as concentrated sulfuric acid orchloro-sulfonic acid. A suitable shape for the polymer is a cylindricalrod which, after sulfonation and conversion to a suitable metal ionform, can be placed in the bore of a chromatography column typically 4mm in internal diameter. Preferably, the ion exchange rod is introducedinto the column in a slightly shrunken form so that in its typical useenvironment it swells to form a tight fit with the wall of the column.Excess rod is trimmed from the end of the column which is then equippedwith porous platinum electrodes and end fittings. Such a column is nowready for use as a base generator in a chromatography application or asa base generator and a suppressor for the determination of anions by ionchromatography.

EXAMPLE 8

Film-type Bed Generator

In this example, a film-type ion exchange bed is formed. A strip ofcation exchange membrane in an appropriate metal ion form is rolled onto a solid spindle whose diameter is approximately 5 mm. The width ofthe film and the length of the spindle arc preferably identical. Enoughfilm is added to the spindle to give a final diameter of approximately15 mm. The film spindle assembly is then placed in a snug fit within ahollow cylinder of the same length as the spindle and this assembly isequipped with porous platinum electrodes and end fittings. The rollingof the membrane film should be loose enough to permit ready flow ofaqueous phase parallel to the film while providing adequate exchange ofions between the film and the aqueous phase and minimum band spreadingwhen used as part of a chromatography system. In order to satisfy theserequirements of flow, favorable ion exchange rates and chromatographicbehavior, an ion exchange membrane with a textured surface may bepreferred over a membrane with a very smooth surface; the texturing willact as a separator holding the membrane sufficiently open to allowadequate aqueous flow while providing the other desirable attributes.

Such a device is now ready for use as a base generator in a typicalchromatography application or as a base generator and a suppressor forthe determination of anions by ion chromatography.

EXAMPLE 9

Comparison of Single-Bed and Dual-Bed KOH Generators

A 4-mm ID dual-bed KOH generator column consisting of a 63-mm length bedof an upstream strongly acidic resin and a 14-mm length bed of adownstream weakly acidic resin was prepared. An 18-μm sulfonatedstyrene/divinylbenzene resin in K⁺ form was used as the strongly acidicresin. A 10-μm macroporous styrene/divinylbenzene resin withsurface-grafted α-chloroacrylic acid functional groups in K⁺ form wasused as the weakly acidic resin. The resins were directly adjacent andin contact. The anode was at the inlet of the strongly acidic resin andthe cathode at the weakly acid outlet.

As a comparison, a single-bed KOH generator column of the samedimensions (4-mm ID×77-mm length) packed with the 18-μm sulfonatedstyrene/divinylbenzene resin was also prepared. The single-bed anddual-bed KOH generator columns were tested using a constant current of15 mA and a carrier flow rate of 1.0 m/min. The concentration of KOH inthe generator column effluent was monitored by measuring the conductanceof the effluent with a conductivity detector.

The exhaustion profile and operating voltage obtained for the dual-bedKOH generator column are shown in FIGS. 18 and 19. The dual-bed KOHgenerator column produced a relatively constant output of KOH (9.3 mMKOH at 1.0 mL/min) for 96 minutes, equivalent to a useful capacity of0.90 meq KOH (60% utilization capacity). The applied voltage decreasedfrom 145 V at the beginning of the operation to 100 V at 60 min, andthen increased to 400 V at 100 min due to the development of the lessconductive protonated zone in weakly acidic resin bed. The exhaustionprofile and operating voltage obtained for the single-bed KOH generatorcolumn are shown in FIGS. 20 and 21. The single-bed KOH generator columnproduced a relatively constant output of KOH (9.3 mM KOH at 1.0 mL/min)for 72 minutes, equivalent to a useful capacity of 0.67 meq KOH (36%utilization capacity). The applied voltage decreased gradually from 125V at the beginning of the operation to 60 V at 150 minutes. Incomparison, the useful capacity of the dual-bed KOH generator column was34 percent higher than that of the single-bed KOH generator column.Thus, the dual-bed eluent generator column is a viable approach toincrease the useful capacity of the KOH generator column.

What is claimed is:
 1. A method of generating an eluent and using thesame in anion analysis comprising (a) flowing a first aqueous stream ina first direction through a flowthrough first cation exchange bedincluding exchangeable cations and having an inlet portion and an outletportion, while applying an electrical potential between an anode inelectrical communication with the inlet portion of said first cationexchange bed and a cathode in electrical communication with the outletportion of said first cation exchange bed to electrolytically generatehydroxide ions and to electromigrate exchangeable cations into saidfirst aqueous stream to form a first eluent comprising a base, saidfirst cation exchange bed comprising an upstream bed portion comprisinga strongly acidic cation exchange material and an adjacent downstreambed portion comprising a weakly acidic cation exchange material, (b)flowing a first liquid sample stream containing anions to be detectedand said first eluent through a chromatographic separator in which saidanions to be detected are separated, forming a first chromatographyeffluent, and (c) flowing said first chromatography effluent, with orwithout further treatment, past a detector in which the separated anionsin said first chromatography effluent are detected.
 2. The method ofclaim 1 in which said cathode is in said outlet portion and said base isformed in said first cation exchange bed and flows from there throughsaid chromatographic separator.
 3. The method of claim 1 in which saidfirst cation exchange bed comprises a first cation exchange resin packedbed.
 4. The method of claim 1 in which said anode and said cathode arein direct contact with said first cation exchange bed.
 5. The method ofclaim 1 further comprising between steps (b) and (c) the step of (d)flowing said first chromatography effluent through a flowthrough secondcation exchange bed including exchangeable cations and hydronium ions tosubstitute said exchangeable hydronium ions with the cations of saidfirst eluent base to convert said base to weakly ionized form, saidfirst chromatography effluent exiting as a first suppressed effluentwhich flows past said detector.
 6. The method of claim 5 in which saidsecond cation exchange bed comprises a second cation exchange resinpacked bed.
 7. The method of claim 5 in which after step (c) the flow ofsteps (a), (b), (d), (c) is discontinued, said method further comprising(e) flowing a second aqueous stream in a second direction opposite tosaid first direction through said second cation exchange bed, whilepassing an electric current to electrolytically generate and displaceexchangeable cations on said second cation exchange bed into said secondaqueous stream to form a second eluent comprising a base which exitssaid second cation exchange bed, (f) flowing a second sample containinganions to be detected and said second eluent through saidchromatographic separator in which said anions in said second sample areseparated, forming a second chromatography effluent, (g) flowing saidsecond chromatographic effluent in said second direction through saidfirst cation exchange bed further including exchangeable hydronium ionsto convert said second eluent base to weakly ionized form andreplenishing exchangeable cations on said first cation exchange beddepleted during step (a), said second chromatography effluent exiting asa second suppressed effluent, and (h) flowing said second suppressedeffluent past said detector in which the separated anions in said secondsuppressor effluent are detected.
 8. The method of claim 1 in which instep (a) said aqueous stream is electrolyzed during passage of saidelectrical current to generate hydrogen and oxygen gases, said methodfurther comprising (d) pressurizing the first chromatography effluentprior to detection in step (c) to minimize adverse affects of said gaseson detection of said first sample anions.
 9. The method of claim 1 inwhich said exchangeable cations in said first cation exchange resin bedare partially depleted during step (a), said method further comprisingdiscontinuing the flow of said first aqueous stream and flowing aregenerating aqueous stream comprising a base through said first cationexchange bed to at least partially replenish said exchangeable cations.10. The method of claim 1 further comprising after step (c) the step of(d) flowing said first chromatography effluent from said detectorthrough a second cation exchange bed including exchangeable hydroniumions which are substituted by the cations of said first eluent.
 11. Themethod of claim 10 in which after step (d) the flow of steps (a)-(d) isdiscontinued, said method further comprising (e) flowing, an aqueousstream in a second direction opposite to said first direction throughsaid second cation exchange bed including exchangeable cations whilepassing an electric current to electrolytically generate and displacesaid exchangeable cations in said second cation exchange bed into saidsecond aqueous stream to form a second eluent comprising a base whichexits said second cation exchange bed, (f) flowing a second samplecontaining anions to be detected and said second eluent through saidchromatographic separator in which said anions in said second sample areseparated, forming a second chromatographic effluent, (g) flowing saidsecond chromatography effluent through said detector in which theseparated anions in said second chromatography effluent are detected,and (h) flowing said second chromatographic effluent in said seconddirection through said first cation exchange bed including exchangeablehydronium ions which are substituted by the cations of said secondeluent, replenishing exchangeable cations lost on said first cationexchange bed depleted during step (a).
 12. The method of claim 1 inwhich said detection in step (c) is by electrical conductivitydetection.
 13. The method of claim 1 in which said upstream portion hasa total ion exchange capacity at least about 10 times that of saiddownstream bed portion.
 14. A method of generating an eluent and usingthe same in cation analysis comprising (a) flowing a first aqueousstream in a first direction through a flowthrough first anion exchangebed including exchangeable anions and having an inlet portion and anoutlet portion, while applying an electrical potential between a cathodein electrical communication with the outlet portion of said first anionexchange bed and an anode in electrical communication with the outletportion of said first anion exchange bed to electrolytically generatehydronium ions and to electromigrate exchangeable anions into said firstaqueous stream to form a first eluent comprising an acid, said firstanion exchange bed comprising an upstream bed portion comprising astrongly basic anion exchange material and an adjacent downstream bedportion comprising a weakly basic anion exchange material, (b) flowing afirst liquid sample stream containing cations to be detected and saidfirst eluent through a chromatographic separator in which said cationsto be detected are separated, forming a first chromatography effluent,and (c) flowing said first chromatography effluent, with or withoutfurther treatment, past a detector in which the separated cations insaid first chromatography effluent are detected.
 15. The method of claim14 in which said cathode is in said outlet portion and said base isformed in said first cation exchange bed and flows from there throughsaid chromatographic separator.
 16. The method of claim 14 in which saidfirst anion exchange bed comprises a first anion exchange resin packedbed.
 17. The method of claim 14 in which said anode and said cathode arein direct contact with said first anion exchange bed.
 18. The method ofclaim 14 further comprising between steps (b) and (c) the step of (d)flowing said first chromatography effluent through a flowthrough secondanion exchange bed including exchangeable anions and hydroxide ions tosubstitute said exchangeable hydroxide ions with the anions of saidfirst eluent acid to convert said acid to weakly ionized form, saidfirst chromatography effluent exiting as a first suppressed effluentwhich flows past said detector.
 19. The method of claim 18 in which saidsecond anion exchange bed comprises a second anion exchange resin packedbed.
 20. The method of claim 14 in which in step (a) said aqueous streamis electrolyzed during passage of said electrical current to generatehydrogen and oxygen gases, said method further comprising (d)pressurizing the first chromatography effluent prior to detection instep (c) to minimize adverse affects of said gases on detection of saidfirst sample anions.
 21. The method of claim 18 in which after step (c)the flow of steps (a), (b), (d), (c) is discontinued, said methodfurther comprising (e) flowing a second aqueous stream in a seconddirection opposite to said first direction through said second anionexchange bed, while passing an electric current to electrolyticallygenerate and displace exchangeable anions on said second anion exchangebed into said second aqueous stream to form a second eluent comprisingan acid which exits said second anion exchange bed, (f) flowing a secondsample containing cations to be detected and said second eluent throughsaid chromatographic separator in which said cations in said secondsample are separated, forming a second chromatography effluent, (g)flowing said second chromatographic effluent in said second directionthrough said first anion exchange bed further including exchangeablehydroxide ions to convert said second eluent acid to weakly ionized formand replenishing exchangeable anions on said first anion exchange beddepleted during step (a), said second chromatography effluent exiting asa second suppressed effluent, and (h) flowing said second suppressedeffluent past said detector in which the separated cations in saidsecond suppressor effluent are detected.
 22. The method of claim 14 inwhich said exchangeable anions in said first anion exchange resin bedare partially depleted during step (a), said method further comprisingdiscontinuing the flow of said first aqueous stream and flowing aregenerating aqueous stream comprising an acid through said first cationexchange bed to at least partially replenish said exchangeable anions.23. The method of claim 14 further comprising after step (c) the step of(d) flowing said first chromatography effluent from said detectorthrough a second anion exchange bed including exchangeable hydroxideions which are substituted by the anions of said first eluent.
 24. Themethod of claim 23 in which after step (d) the flow of steps (a)-(d) isdiscontinued, said method further comprising (e) flowing an aqueousstream in a second direction opposite to said first direction throughsaid second anion exchange bed including exchangeable anions whilepassing an electric current to electrolytically generate and displacesaid exchangeable anions in said second anions exchange bed into saidsecond aqueous stream to form a second eluent comprising an acid whichexits said second anion exchange bed, (f) flowing a second samplecontaining cations to be detected and said second eluent through saidchromatographic separator in which said cations in said second sampleare separated, forming a second chromatographic effluent, (g) flowingsaid second chromatography effluent through said detector in which theseparated cations in said second chromatography effluent are detected,and (h) flowing said second chromatographic effluent in said seconddirection through said first anion exchange bed including exchangeablehydroxide ions which are substituted by the anions of said secondeluent, replenishing exchangeable anions on said first anion exchangebed during step (a).
 25. The method of claim 14 in which said detectionin step (c) is by electrical conductivity detection.
 26. The method ofclaim 14 in which said upstream portion anion exchange material has acapacity at least about 10 times that of said downstream bed portionanion exchange material.